1. Technical Field
The present invention relates to an electromagnet for bending an orbit of a charged particle beam and a magnetic field generating apparatus for generating a magnetic field in an air gap and, in particular to an electromagnet suitable for use in a physics experimentation accelerator, an industrial accelerator and a medical treatment accelerator or a magnetic field generating apparatus for use in a medical treatment diagnostic instrument such as MRI (Magnetic Resonance Imager).
2. Prior Arts
In general, an electromagnet used in an accelerator is designed to have a desirable magnetic field distribution in a predetermined area which is determined by the size of a charged particle beam (hereinafter, a beam). However, in a case when a magnetic field strength increases, due to the effect of magnetic saturation of the iron core which is used in the magnetic pole of the electromagnet, the leakage magnetic field from the magnetic pole increases. According to the effect of this leakage magnetic field, since an area (hereinafter, a preferable magnetic field area) becomes narrow, where an amount of displacement from a desirable magnetic field distribution is less than a predetermined value, the electromagnet""s maximum magnetic field strength that is actually useable is limited.
For example, for a bending electromagnet used in an accelerator, when a radius of curvature and a maximum magnetic field strength are determined, a maximum energy of the beam obtained by the accelerator is determined. As a result, when a beam with a wide energy range is sought, since the maximum magnetic field strength of the electromagnet is limited for the above stated reasons, it is necessary to employ a bending electromagnet having a large radius of curvature.
As to the prior art, a bending electromagnet is disclosed in Japanese patent laid-open publication No. Hei 5-47, 547 (hereinafter first prior art). In such a bending electromagnet, a projection which is a magnetic pole shim is provided at an end portion of the magnetic poles which are arranged oppositely.
Further, recently attention has been paid to a medical treatment use diagnosing instrument using a magnetic field such as an open type MRI, in which a permanent magnet having a flexibility is utilized, and also about a method for generating a high magnetic field and a good uniformity magnetic field in an air gap.
In the magnetic field generating apparatus having a permanent magnet, a conventional technique for generating the magnetic field having a high magnetic field and the good uniformity is disclosed in Japanese patent laid-open publication No. Hei 5-243,037 (hereinafter, the second prior art). This magnetic field generating apparatus comprises pole pieces which are arranged oppositely and at an end portion a magnetic pole shim is provided, and permanent magnets.
Further, this magnetic field apparatus has a central portion of the pole piece which is moved toward an upper portion and a lower portion. A lateral cross-section of an upper portion magnetic pole portion in this magnetic field generating apparatus is shown in FIG. 12. In FIG. 12, 101 denotes a permanent magnet, 103 denotes a yoke, 108 denotes a permanent magnet fixing use bolt, and 121 and 122 denote pole pieces.
In the above stated first prior art, by concentrating a line of magnetic force which spreads from an end portion of the magnetic pole toward a lateral direction, it aims to spread a good field region (stated in a latter portion) formed between the magnetic poles.
However, since it is necessary to provide the projection to the magnetic pole, a magnetic pole structure becomes a complicated one. Further, since the magnetic field distribution formed between the magnetic poles is affected largely by a shape of the projection, high processing accuracy for manufacturing the magnetic poles is required. Also, a manufacturing cost becomes high.
Further, in the above stated second prior art, similarly to the first prior art, since it is necessary to provide the projection to the pole piece, the pole piece structure becomes a complicated one. Further, since the magnetic field distribution formed between the pole pieces is affected largely by a shape of the projection, high processing accuracy for the pole pieces is required.
Further, in the second prior art, since a movement mechanism is provided, a large mechanical force is applied to a mechanical sliding portion, and a high reliability, and a high reproducibility are required. Also, a manufacturing cost becomes high.
On the other hand, in the deflecting electromagnet in which a projection is not provided to a magnetic pole, since a maximum magnetic field strength is limited according to the affect of magnetic saturation, in a case where a beam with a wide energy range is sought, the bending electromagnet becomes a large size one and also the accelerator becomes a large size one. Similarly too, the magnetic field generating apparatus in which the projection is not provided at the pole piece becomes a large size one.
A first object according to the present invention is to provide an electromagnet which has a simple structure and secures a high magnetic field and a good uniformity magnetic field or a magnetic field generating apparatus. Herein, the good magnetic field having the uniformity property indicates that the effective magnetic field area is wide. Further, the effective magnetic field area is one which can be used for bending an orbit of a beam in a preferable magnetic field area.
A second object according to the present invention is to provide an accelerator which has a wide beam energy range and a small size structure.
A first embodiment for attaining the first object is an electromagnet for bending an orbit of a charged particle beam according to a magnetic field which is generated between magnetic poles, said magnetic pole having at an interior portion thereof an area which has a lower relative permeability than that of a magnetic pole forming material.
A second embodiment for attaining the first object is an electromagnet that comprises at least one pair of magnetic poles which are arranged oppositely, a return yoke for forming a magnetic path by connecting said magnetic poles to each other, and a coil for generating a magnetic field to vary an orbit of a charged particle beam in a space formed between said magnetic poles, said magnetic pole having a space at an interior portion thereof.
A third embodiment for attaining the first object is an electromagnet that comprises at least one pair of magnetic poles which are arranged oppositely, a return yoke for forming a magnetic path by connecting said magnetic poles each other, and a coil for generating a magnetic field in a space to vary an orbit of a charged particle beam between said magnetic poles, a space being provided at an interior portion of said magnetic pole to form substantially homogeneously a magnetic field strength at a predetermined area formed between said magnetic poles.
A fourth embodiment for attaining the second object is an accelerator that comprises a first half accelerator for generating a charged particle beam, a synchrotron for increasing an energy by accelerating said charged particle beam, and a beam transportation system which injects said charged particle beam extracted from said first half accelerator to said synchrotron, said synchrotron having an electromagnet with an area having a lower relative permeability than that of a magnetic pole forming material is provided.
A fifth embodiment for attaining the second object is an accelerator that comprises a first half accelerator for generating a charged particle beam, a synchrotron for increasing an energy by accelerating said charged particle beam, and a beam transportation system which injects said charged particle beam extracted from said first half accelerator to said synchrotron, said synchrotron having an electromagnet in which a space is provided at an interior portion of a magnetic pole to form substantially homogeneously a magnetic field strength at a predetermined area formed between said magnetic poles.
A sixth embodiment for attaining the first object is a magnetic field generating apparatus that comprises at least one pair of pole pieces which are arranged oppositely by forming an air gap, and a permanent magnet for supplying a magnetic flux to said pole piece, said pole piece having at an interior portion thereof an area which has a lower relative permeability than that of a pole piece forming material.